Sheet conveyance apparatus and image forming apparatus

ABSTRACT

A sheet conveyance apparatus includes a connection member to connect or separate a motor to drive a conveyance roller, where a motor rotor has a rotation phase and a motor winding includes a driving current controlled by a controller. Per control by the controller, a value of a torque current component of a detected driving current becomes a target value of the torque current component and a value of an exciting current component of the detected driving current becomes a target value of the exciting current component. The controller can cause a deviation between a rotor target phase and a determined rotation phase to decrease, cause magnetic flux passing through the winding to be weaker than a magnetic flux of the rotor, and cause the magnetic flux to be stronger than that passing through the winding in a first period.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation, and claims the benefit of, U.S.patent application Ser. No. 15/873674, filed Jan. 17, 2018, which claimsthe benefit of Japanese Patent Application No. 2017-022465, filed Feb.9, 2017, all of which are hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to controlling a motor in a sheetconveyance apparatus and an image forming apparatus.

Description of the Related Art

A control method called vector control has heretofore been known as amethod for controlling a motor. Vector control controls a motor bycontrolling a current value in a rotating coordinate system based on arotation phase of a rotor of the motor. Specifically, there has beenknown a method for controlling a motor by phase feedback control inwhich a current value is controlled in the rotating coordinate system insuch a manner that a deviation between a command phase and the actualrotation phase of the rotor decreases. There has also been known amethod for controlling a motor by speed feedback control in which acurrent value is controlled in the rotating coordinate system in such amanner that a deviation between a command speed and the actual rotationspeed of the rotor decreases.

In vector control, a driving current supplied to a winding of the motoris expressed by a current component (torque current component) thatgenerates torque for rotating the rotor and a current component(exciting current component) that affects an intensity of a magneticflux passing through the winding. Torque needed for rotation isefficiently generated by controlling the value of the torque currentcomponent according to a change in load torque acting on the rotor. Thissuppresses an increase in motor noise and an increase in powerconsumption due to surplus torque. This also prevents the rotor fromgoing out of synchronization with an input signal and the motor fromentering an incontrollable state (step-out state) because the loadtorque acting on the rotor exceeds output torque corresponding to thedriving current supplied to the winding of the motor. During vectorcontrol, the value of the exciting current component is usuallycontrolled so as to become zero. As a result, an increase in powerconsumption is suppressed.

Rotation of the rotor generates an induced voltage in the winding ofeach phase of the motor. If an induce voltage occurs in the windings ofthe motor, the voltage applicable to the windings of the motordecreases. Specifically, suppose, for example, that a power supply forapplying a voltage to the windings of the motor has a voltage of 24 V.In such a case, a voltage obtained by subtracting the induced voltageoccurring in the windings from the power supply voltage (24 V) can beapplied to the windings. The occurrence of the induced voltage in thewindings thus makes the voltage applicable to the windings lower than 24V. The magnitude of the induced voltage increases when the rotationspeed of the rotor increases. The higher the rotation speed of therotor, the lower the voltage applicable to the windings of the motor. Asthe voltage applicable to the windings of the motor decreases, torquethat can be given to the rotor (hereinafter, referred to as torque T)also decreases.

Japanese Patent Application Laid-Open No. 2007-153273 discusses aconfiguration for weakening an intensity of a magnetic field passingthrough the windings of the motor (field weakening) by controlling thevalue of the exciting current component to a negative valuecorresponding to the rotation speed of the rotor if the rotation speedis a speed threshold or more. There is a one-to-one correspondencebetween the rotation speed and the value of the excitation currentcomponent. Specifically, a predetermined value of the exciting currentcomponent is set with respect to a predetermined rotation speed. Fieldweakening reduces the magnitude of the induced voltage occurring in thewindings. This can suppress a decrease in the voltage applicable to thewindings and can prevent the torque T from decreasing. The greater theabsolute value of the negative value of the exciting current component,the more the torque T can be prevented from decreasing.

A sheet conveyance apparatus for conveying a sheet, such as a recordingmedium or a document, includes a plurality of loads like rollers forconveying the sheet. A plurality of loads is sometimes driven by onemotor. In such a case, for example, the loads are driven by connectingand separating the motor and the loads by a clutch.

FIG. 1 is a block diagram illustrating a configuration of a motor andconveyance rollers serving as loads. As illustrated in FIG. 1, aconveyance roller 701 is driven by a motor 509. The motor 509 and aconveyance roller 702 are connected and separated by a clutch 700.

The clutch 700 connects and separates the motor 509 and the conveyanceroller 702 in a state in which the rotor of the motor 509 is rotating ata predetermined speed (constant speed). In other words, a period inwhich the rotor of the motor 509 rotates at the predetermined speedincludes a period during which the motor 509 and the conveyance roller702 are not connected and a period during which the motor 509 and theconveyance roller 702 are connected. If the motor 509 is driving theconveyance roller 701 in such a manner that the motor 509 and theconveyance roller 702 are separated, load torque corresponding to theconveyance roller 701 acts on the rotor of the motor 509. If the motor509 and the conveyance roller 702 are connected while the motor 509 isdriving the conveyance roller 701, not only the load torquecorresponding to the conveyance roller 701 but also load torquecorresponding to the conveyance roller 702 acts on the rotor of themotor 509. The load torque acting on the rotor thus increases when themotor 509 and the conveyance roller 702 are connected by the clutch 700.As the loads connected to the motor 509 increase, the load torque actingon the rotor of the motor 509 in rotating the rotor at a predeterminedspeed increases. In the period in which the rotor rotates at apredetermined speed, the torque T therefore decreases due to the inducedvoltage occurring in the windings. In the period in which the rotorrotates at a predetermined speed, the load torque acting on the rotorcan thus exceed the torque T due to the connection of the loads to themotor 509. If the load torque exceeds the torque T, the rotor becomesunable to rotate.

In the configuration discussed in Japanese Patent Application Laid-OpenNo. 2007-153273, the rotation speed and the value of the excitingcurrent component have a one-to-one correspondence. In the period inwhich the rotor rotates at a predetermined speed, the exciting currentcomponent is therefore set to a predetermined value corresponding to thepredetermined speed.

As described above, in the period in which the rotor rotates at apredetermined speed, the load torque during the period in which themotor and a load are connected is higher than that during the period inwhich the motor and the load are not connected. In other words, if theconfiguration discussed in Japanese Patent Application Laid-Open No.2007-153273 is applied to the control of the motor that is connected toand separated from a load by the clutch, the value of the excitingcurrent component needs to be set in consideration of the load torqueduring the period in which the motor and the load are connected, in sucha manner that the load torque will not exceed the torque T.

The larger the value of the exciting current component in absolutevalue, the greater the current supplied to the windings of the motor. Ifthe value of the exciting current component is set in consideration ofthe load torque during the period in which the motor and the load areconnected, an unneeded current can be supplied to the windings duringthe period in which the motor and the load are not connected. As aresult, the power consumption increases.

SUMMARY OF THE INVENTION

The present disclosure is directed to efficiently performing motorcontrol.

According to an aspect of the present invention, a sheet conveyanceapparatus for conveying a sheet includes a conveyance roller configuredto convey the sheet, a motor configured to drive the conveyance roller,a connection member configured to connect or separate the conveyanceroller and the motor, a phase determiner configured to determine arotation phase of a rotor of the motor, a detector configured to detecta driving current flowing through a winding of the motor, and acontroller configured to control the driving current in such a way thata value of a torque current component of the driving current detected bythe detector becomes a target value of the torque current component, andcontrol the driving current in such a way that a value of an excitingcurrent component of the driving current detected by the detectorbecomes a target value of the exciting current component, wherein thetorque current component is a current component that generates torque onthe rotor and is expressed in a rotating coordinate system based on therotation phase determined by the phase determiner, wherein the excitingcurrent component is a current component that affects an intensity of amagnetic flux passing through the winding and is expressed in therotating coordinate system, wherein the controller is configured to setthe target value of the torque current component such that a deviationbetween a command phase indicating a target phase of the rotor and therotation phase determined by the phase determiner decreases and, in afirst period after first timing in a period in which the rotor rotatesat a predetermined speed, set the target value of the exciting currentcomponent such that the magnetic flux passing through the windingbecomes weaker than a magnetic flux of the rotor and, in a second periodbefore the first timing, set the target value of the exciting currentcomponent such that the magnetic flux passing through the windingbecomes stronger than that passing through the winding in the firstperiod, wherein the first timing is a predetermined time before theconveyance roller and the motor are connected by the connection member.

Further features of the present invention will become apparent from thefollowing description of embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a motor andloads.

FIG. 2 is a sectional view for describing an image forming apparatusaccording to a first embodiment.

FIG. 3 is a block diagram illustrating a control configuration of theimage forming apparatus.

FIG. 4 is a diagram illustrating a relationship of a motor having twophases, an A phase and a B phase, with a d-axis and a q-axis of arotating coordinate system.

FIG. 5 is a block diagram illustrating a configuration of a motorcontrol device according to the first embodiment.

FIG. 6 is a diagram illustrating a configuration of a full-bridgecircuit included in a pulse width modulation (PWM) inverter.

FIG. 7 is a graph illustrating a relationship between torque T and arotation speed of a rotor.

FIG. 8 is a diagram illustrating a time chart of field weakening controlaccording to the first embodiment.

FIG. 9 is a flowchart for describing a method of the field weakeningcontrol according to the first embodiment.

FIG. 10 is a block diagram illustrating a configuration of a motorcontrol device that performs speed feedback control.

DESCRIPTION OF THE EMBODIMENTS

An embodiment will be described below with reference to the drawings.Shapes and a relative arrangement of components described in theembodiment are subject to appropriate changes according to aconfiguration and various conditions of an apparatus to which theembodiment is applied. The scope is not to be limited to the followingembodiment. In the following description, a motor control device isdescribed to be provided in an image forming apparatus. However, theprovision of the motor control device is not limited to an image formingapparatus. For example, the motor control device may be used in a sheetconveyance apparatus for conveying a sheet such as a recording medium ora document.

[Image Forming Apparatus]

FIG. 2 is a sectional view illustrating a configuration of a monochromeelectrophotographic copying machine (hereinafter, referred to as animage forming apparatus) 100 which includes a sheet conveyance apparatusused in a first embodiment. The image forming apparatus is not limitedto a copying machine. For example, the image forming apparatus may be afacsimile apparatus, a printing machine, or a printer. The recordingmethod is not limited to an electrophotographic method. For example, therecording method may be an inkjet method. The image forming apparatusmay be of either a monochrome or color model.

The configuration and functions of the image forming apparatus 100 willbe described below with reference to FIG. 2. The image forming apparatus100 includes a document feeding apparatus 201, a reading apparatus 202,and an image printing apparatus 301.

Documents stacked on a document stacking unit 203 of the documentfeeding apparatus 201 are fed by feed rollers 204 one by one, andconveyed along a conveyance guide 206 onto a document glass plate 214 ofthe reading apparatus 202. The document is further conveyed by aconveyance belt 208 at a constant speed, and discharged to a dischargetray (not illustrated) by discharge rollers 205. Reflected light from adocument image irradiated by an illumination system 209 at a readingposition of the reading apparatus 202 is guided to an image reading unit111 by an optical system including reflection mirrors 210, 211, and 212,and converted into an image signal by the image reading unit 111. Theimage reading unit 111 includes a lens, a charge-coupled device (CCD)which is a photoelectric conversion element, and a driving circuit ofthe CCD. An image processing unit 112 including a hardware device suchas an application specific integrated circuit (ASIC) performs varioustypes of correction on the image signal output from the image readingunit 111. The resulting image signal is output to the image printingapparatus 301. In such a manner, the document is read. That is, thedocument feeding apparatus 201 and the reading apparatus 202 function asa document reading apparatus.

Reading modes of a document include a first reading mode and a secondreading mode. In the first reading mode, an image of a document conveyedat a constant speed is read by the illumination system 209 and theoptical system that are fixed in a predetermined position. In the secondreading mode, an image of a document placed on the document glass plate214 of the reading apparatus 202 is read by the illumination system 209and the optical system that move at a constant speed. An image of asheet-like document is usually read in the first reading mode. An imageof a bound document, such as a book and a booklet, is read in the secondreading mode.

The image printing apparatus 301 includes sheet storage trays 302 and304 inside. The sheet storage trays 302 and 304 can store respectivedifferent types of recording media. For example, A4-size plain paper isstored in the sheet storage tray 302. A4-size thick paper is stored inthe sheet storage tray 304. Recording media refer to ones on which theimage forming apparatus 100 forms an image. Examples of the recordingmedia include a sheet of paper, a resin sheet, cloth, an overheadprojector (OHP) sheet, and a label.

The recording media stored in the sheet storage tray 302 is fed by afeed roller 303 and delivered to a registration roller 308 by aconveyance roller 306. The recording media stored in the sheet storagetray 304 are fed by a feed roller 305 and delivered to the registrationroller 308 by a conveyance roller 307 and the conveyance roller 306. Asillustrated in FIG. 2, sheet sensors 330 and 331 for detecting thepresence or absence of a recording medium are provided upstream anddownstream of the conveyance roller 306. The uses of the sheet sensors330 and 331 will be described below. In the present embodiment, thesheet sensors 330 and 331 are optical sensors. However, this is notrestrictive. For example, the sheet sensors 330 and 331 may be flagsensors.

The image signal output from the reading apparatus 202 is input to anoptical scanning device 311 which includes a semiconductor laser and apolygonal mirror. An outer peripheral surface of a photosensitive drum309 is charged by a charging device 310. After the outer peripheralsurface of the photoelectric drum 309 is charged, laser light accordingto the image signal input from the reading apparatus 202 to the opticalscanning device 311 is projected from the optical scanning device 311onto the outer peripheral surface of the photoelectric drum 309 via thepolygonal mirror, a mirror 312, and a mirror 313. As a result, anelectrostatic latent image is formed on the outer peripheral surface ofthe photosensitive drum 309.

The electrostatic latent image is then developed with toner in adeveloping device 314, whereby a toner image is formed on the outerperipheral surface of the photosensitive drum 309. The toner imageformed on the photosensitive drum 309 is transferred to a recordingmedium by a transfer charging device 315 which is arranged in a position(transfer position) opposed to the photosensitive drum 309. Theregistration roller 308 feeds the recording medium into the transferposition in synchronization with timing at which the transfer chargingdevice 315 transfers the toner image to the recording medium.

The recording medium to which the toner image is transferred asdescribed above is fed into a fixing device 318 by a conveyance belt317. The fixing device 318 applies heat and pressure to the recordingmedium, whereby the toner image is fixed to the recording medium. Insuch a manner, the image forming apparatus 100 forms an image on arecording medium.

If an image is formed in a one-sided printing mode, the recording mediumhaving passed through the fixing device 318 is discharged to a dischargetray (not illustrated) by discharge rollers 319 and 324. If an image isformed in a two-sided printing mode, the recording medium on a firstside of which fixing is performed by the fixing device 318 is thenconveyed to a reversing path 325 by the discharge roller 319, aconveyance roller 320, and a reversing roller 321. The recording mediumis then conveyed to the registration roller 308 again by conveyancerollers 322 and 323, and an image is formed on a second side of therecording medium by the foregoing method. The recording medium is thendischarged to the discharge tray (not illustrated) by the dischargerollers 319 and 324.

If the recording medium having an image formed on its first side isdischarged out of the image forming apparatus 100 facedown, therecording medium having passed through the fixing device 318 passesthrough the discharge roller 319 and is conveyed toward the conveyanceroller 320. Immediately before the trailing edge of the recording mediumpasses through a nip portion of the conveyance roller 320, rotation ofthe conveyance rollers 320 is reversed. The recording medium is therebydischarged out of the image forming apparatus 100 via the dischargeroller 324 with the first side of the recording medium down.

The configuration and functions of the image forming apparatus 100 havebeen described above. In the present embodiment, a load refers to anobject to be driven by a motor. For example, various rollers (conveyancerollers) including the feed rollers 204, 303, 305, the registrationroller 308, and the discharge roller 319, the photosensitive drum 309,the conveyance belts 208 and 317, the illumination system 209, and theoptical system correspond to loads according to the present embodiment.The motor control device according to the present embodiment may beemployed for the motors for driving such loads.

FIG. 3 is a block diagram illustrating an example of a controlconfiguration of the image forming apparatus 100. As illustrated in FIG.3, the image forming apparatus 100 includes a power supply 1. The powersupply 1 is connected to an alternating-current (AC) power source.Various devices in the image forming apparatus 100 operate depending onpower output from the power supply 1. As illustrated in FIG. 3, a systemcontroller 151 includes a central processing unit (CPU) 151 a, aread-only memory (ROM) 151 b, and a random access memory (RAM) 151 c.The system controller 151 is connected with an image processing unit112, an operation unit 152, an analog-to-digital (A/D) converter 153, ahigh-voltage control unit 155, a motor control device 157, a clutch 700,the sheet sensors 330 and 331, sensors 159, and an AC driver 160. Thesystem controller 151 can transmit and receive data and commands to/fromthe connected units.

The CPU 151 a reads and executes various programs stored in the ROM 151b, and thereby executes various sequences related to a predeterminedimage forming sequence.

The RAM 151 c is a storage device. The RAM 151 c stores various types ofdata, such as a setting value for the high-voltage control unit 155, acommand value for the motor control device 157, and information receivedfrom the operation unit 152.

The system controller 151 transmits setting value data on variousdevices included in the image forming apparatus 100 to the imageprocessing unit 112. The setting value data is needed for imageprocessing of the image processing unit 112. The system controller 151further receives signals from the sensors 159, and sets the settingvalue of the high-voltage control unit 155 based on the receivedsignals.

The high-voltage control unit 155 reads the setting value set by thesystem controller 151 from the RAM 151 c, and supplies a needed voltageto a high-voltage unit 156 (such as the charging device 310, thedeveloping device 314, and the transfer charging device 315).

As illustrated in FIG. 3, a motor 509 according to the presentembodiment drives a plurality of loads. Specifically, for example, themotor 509 drives the conveyance roller 307 and the conveyance roller306. The motor 509 and the conveyance roller 306 are connected andseparated by the clutch 700. In other words, the clutch 700 functions asa connection member. If the motor 509 and the conveyance roller 306 areconnected by the clutch 700, the motor 509 can drive the conveyancerollers 307 and 306. If the motor 509 and the conveyance roller 306 areseparated, the motor 509 drives only the conveyance roller 307. Theconnection and separation of the clutch 700 are performed in a statewhere a rotor of the motor 509 rotates at a predetermined speed(constant speed). The clutch 700 according to the present embodiment isan electromagnetic clutch which performs connection and separation byelectromagnetic force. However, this is not restrictive. The clutch 700may have any configuration for connecting and separating the motor 509and the load to transmit the driving force of the motor 509 to the load.

The system controller 151 controls the clutch 700 based on detectionresults of the sheet sensors 330 and 331. The clutch 700 connects andseparates the motor 509 and the conveyance roller 306 according to asignal output from the CPU 151 a.

The motor control device 157 controls the motor 509 according to acommand output from the CPU 151 a. In FIG. 3, the motor 509 isconfigured to drive the conveyance rollers 306 and 307. However, this isnot restrictive. For example, the motor 509 may be configured to drivenot only the conveyance roller 306 and 307 but also other loads. In FIG.3, only the motor 509 is illustrated as a motor for driving loads. Infact, the image forming apparatus 100 includes a plurality of motors.One motor control device may be configured to control a plurality ofmotors. While there is provided only one motor control device in FIG. 3,the image forming apparatus 100 may in fact include a plurality of motorcontrol devices.

The power supply 1 supplies a voltage Vcc to a full-bridge circuit 50included in the motor control device 157. The full-bridge circuit 50will be described below.

The A/D converter 153 receives a signal detected by a thermistor 154 fordetecting temperature of a fixing heater 161. The A/D converter 153converts the detection signal from an analog signal into a digitalsignal, and transmits the digital signal to the system controller 151.The system controller 151 controls the AC driver 160 based on thedigital signal received from the A/D converter 153. The AC driver 160controls the fixing heater 161 such that the fixing heater 161 hastemperature needed to perform fixing. The fixing heater 161 is used forfixing and is included in the fixing device 318.

The system controller 151 controls the operation unit 152 to display anoperation screen on a display unit provided on the operation unit 152. Auser sets in the operation screen a type (sheet type) of recordingmedium to be used. The system controller 151 receives information set bythe user from the operation unit 152, and controls an operation sequenceof the image forming apparatus 100 based on the information set by theuser. The system controller 151 transmits information indicating a stateof the image forming apparatus 100 to the operation unit 152. Examplesof the information indicating the state of the image forming apparatus100 include information about the number of images to be formed, a stateof progress of an image forming operation, and a sheet jam or doublefeeding in the image printing apparatus 301 and the document feedingapparatus 201. The operation unit 152 displays the information receivedfrom the system controller 151 on the display unit.

In such a manner, the system controller 151 controls the operationsequence of the image forming apparatus 100.

[Motor Control Device]

Next, the motor control device 157 according to the present embodimentwill be described. The motor control device 157 according to the presentembodiment controls the motor 509 by vector control.

<Vector Control>

A method by which the motor control device 157 according to the presentembodiment performs vector control will initially be described withreference to FIGS. 4 and 5. In the following description, the motor 509is described to include no sensor such as a rotary encoder for detectinga rotation phase of the rotor of the motor 509. However, the motor 509may be configured to include a sensor such as a rotary encoder.

FIG. 4 is a diagram illustrating a relationship between a stepping motor(hereinafter, referred to as a motor) 509 having two phases, an A phase(first phase) and a B phase (second phase), and a rotating coordinatesystem expressed by a d-axis and a q-axis. In FIG. 4, an α-axis and aβ-axis are defined in a stationary coordinate system. The α-axiscorresponds to the windings of the A phase. The β-axis corresponds tothe windings of the B phase. In FIG. 4, the d-axis is defined along adirection of a magnetic flux generated by the poles of a permanentmagnet used in a rotor 402. The q-axis is defined along a direction 90°leading the d-axis counterclockwise (direction orthogonal to thed-axis). An angle formed between the α-axis and the d-axis is defined asθ. The rotation phase of the rotor 402 is expressed by the angle θ. Thevector control uses a rotating coordinate system with reference to therotation phase θ of the rotor 402. Specifically, the vector control usescurrent components, in the rotating coordinate system, of a currentvector corresponding to the driving currents flowing through thewindings, namely, a q-axis component (torque current component) whichgenerates torque on the rotor 402 and a d-axis component (excitingcurrent component) which affects the intensity of a magnetic fluxpassing through the windings.

Vector control is a method of controlling a motor by phase feedbackcontrol of controlling the value of the torque current component andthat of the exciting current component such that a deviation between acommand phase indicating the target phase of the rotor and an actualrotation phase decreases. Another method of controlling a motor is toperform speed feedback control of controlling the value of the torquecurrent component and that of the exciting current component such that adeviation between a command speed indicating the target speed of therotor and an actual rotation speed decreases.

FIG. 5 is a block diagram illustrating an example of a configuration ofthe motor control device 157 which controls the motor 509. The motorcontrol device 157 includes at least one ASIC and performs functions tobe described below.

As illustrated in FIG. 5, the motor control device 157 includes a phasecontroller 502, a current controller 503, an inverse coordinateconverter 505, a coordinate transformer 511, and a pulse widthmodulation (PWM) inverter 506 as circuits for vector control. The PWMinverter 506 supplies the driving currents to the windings of the motor509. The coordinate converter 511 converts the coordinates of thecurrent vector corresponding to the driving currents flowing through thewindings of the A and B phases of the motor 509 from the stationarycoordinate system expressed by the α- and β-axes into the rotatingcoordinate system expressed by the q- and d-axes. As a result, thedriving currents flowing through the winding are expressed by thecurrent value of the q-axis component (q-axis current) and the currentvalue of the d-axis component (d-axis current), which are current valuesin the rotating coordinate system. The q-axis current corresponds to thetorque current that generates torque on the rotor 402 of the motor 509.The d-axis current corresponds to the exciting current that affects theintensity of the magnetic flux passing through the windings of the motor509, and does not contribute to generation of torque on the rotor 402.The motor control device 157 can control each of the q- and d-axescurrents independently. The motor control device 157 can thusefficiently generate torque needed for the rotor 402 to rotate bycontrolling the q-axis current according to load torque acting on therotor 402. In other words, in the vector control, the magnitude of thecurrent vector illustrated in FIG. 3 changes depending on the loadtorque acting on the rotor 402.

The motor control device 157 determines the rotation phase θ of therotor 402 of the motor 509 by a method to be described below, andperforms vector control based on the determination result. The CPU 151 agenerates a command phase θ_ref indicating the target phase of the rotor402 of the motor 509, and outputs the command phase θ_ref to the motorcontrol device 157.

A subtractor 101 calculates a deviation between the rotation phase θ ofthe rotor 402 of the motor 509 and the command phase θ_ref, and outputsthe deviation to the phase controller 502 at predetermined time periodsT (for example, 200 μs).

The phase controller 502 generates and outputs a q-axis current commandvalue (target value) iq_ref so that the deviation output from thesubtractor 101 decreases, based on proportional (P) control, integral(I) control, and derivative (D) control. Specifically, the phasecontroller 502 generates and outputs the q-axis current command valueiq_ref such that the deviation output from the subtractor 101 becomeszero, based on P control, I control, and D control. P control refers toa method of controlling the value to be controlled based on a valueproportional to a deviation between the command value and an estimatedvalue. I control refers to a method of controlling the value to becontrolled based on a value proportional to a time integral of thedeviation between the command value and the estimated value. D controlrefers to a method of controlling the value to be controlled based on avalue proportional to a temporal change of the deviation between thecommand value and the estimated value. In the present embodiment, thephase controller 502 generates the q-axis current command value iq_refbased on PID control. However, this is not restrictive. For example, thephase controller 502 may generate the q-axis current command valueiq_ref based on PI control.

The driving currents flowing through the windings of the A and B phasesof the motor 509 are detected by current detectors 507 and 508, and thenconverted from analog values into digital values by an A/D converter510. In the present embodiment, for example, the A/D converter 510outputs the digital values at periods (e.g., 25 microseconds (μs))shorter than the periods T at which the subtractor 101 outputs thedeviation to the phase controller 502. However, this is not restrictive.

The current values of the driving currents, converted from analog valuesinto digital values by the A/D converter 510, are expressed as currentvalues iα and iβ in the stationary coordinate system by the followingequations, using a phase θe of the current vector illustrated in FIG. 4:

iα=I*cos θe, and  (1)

iβ=I* sin θe.  (2)

The phase θe of the current vector is defined as an angle formed betweenthe α-axis and the current vector. I represents the magnitude of thecurrent vector.

The current values iα and iβ are input to the coordinate converter 511and an induced voltage determiner 512.

The coordinate converter 511 converts the current values iα and iβ inthe stationary coordinate system into a current value iq of the q-axiscurrent and a current value id of the d-axis current in the rotatingcoordinate system by the following equations:

id=cos θ*iα+sin θ*iβ, and  (3)

iq=−sin θ*iα+cos θ*iβ.  (4)

The q-axis current command value iq_ref output from the phase controller502 and the current value iq output from the coordinate converter 511are input to a subtractor 102. The subtractor 102 calculates a deviationbetween the q-axis current command value iq_ref and the current valueiq, and outputs the deviation to the current controller 503.

A d-axis current command value (target value) id_ref output from a fieldcontroller 540 and the current value id output from the coordinateconverter 511 are input to a subtractor 103. The subtractor 103calculates a deviation between the d-axis current command value id_refand the current value id, and outputs the deviation to the currentcontroller 503. The field controller 540 will be described below.

The current controller 503 generates driving voltages Vq and Vd suchthat both the input deviations decrease, based on PID control.Specifically, the current controller 503 generates the driving voltagesVq and Vd such that both the deviations become zero, and outputs thedriving voltages Vq and Vd to the inverse coordinate converter 505. Inother words, the current controller 503 functions as a generation unit.In the present embodiment, the current controller 503 generates thedriving voltages Vq and Vd based on PID control. However, this is notrestrictive. For example, the current controller 503 may generate thedriving voltages Vq and Vd based on PI control.

The inverse coordinate converter 505 inversely converts the drivingvoltages Vq and Vd in the rotating coordinate system, output from thecurrent controller 503, into driving voltages Vα and Vβ in thestationary coordinate system by the following equations:

Vα=cos θ*Vd−sin θ*Vq, and  (5)

Vβ=sin θ*Vd+cos θ*Vq.  (6)

The inverse coordinate converter 505 outputs the inversely-converteddriving voltages Vα and Vβ to the induced voltage determiner 512 and thePWM inverter 506.

The PWM inverter 506 includes the full-bridge circuit 50. FIG. 6 is adiagram illustrating an example of a configuration of the full-bridgecircuit 50 included in the PWM inverter 506. As described above, thevoltage Vcc is supplied from the power supply 1 to the full-bridgecircuit 50. The full-bridge circuit 50 includes field-effect transistors(FETs) Q1 to Q4 serving as switching elements. A winding L1 of the motor509 is connected to the full-bridge circuit 50.

The FETs Q1 to Q4 are driven by PWM signals based on the drivingvoltages Vα and Vβ input from the inverse coordinate converter 505. As aresult, a voltage is applied to the winding L1 from the power supply 1.The driving currents iα and iβ according to the driving voltages Vα andVβ are thus supplied to the winding L1. In other words, the PWM inverter506 functions as a supply unit. In the present embodiment, the PWMinverter 506 includes the full-bridge circuit 50. However, the PWMinverter 506 may include a half-bridge circuit. The full-bridge circuit50 is provided for each of the A and B phases of the motor 509. In thepresent embodiment, one power supply is provided for each of the A and Bphases. However, this is not restrictive. The winding L1 in FIG. 6 is infact a winding included in the motor 509.

Next, a method of determining the rotation phase θ will be described.The rotation phase θ of the rotor 402 is determined by using the valuesof induced voltages Eα and Eβ which are induced in the windings of the Aand B phases of the motor 509 by rotation of the rotor 402. The valuesof the induced voltages Eα and Eβ are determined (calculated) by theinduced voltage determiner 512. Specifically, the induced voltages Eαand Eβ are determined based on the current values iα and iβ input fromthe A/D converter 510 to the induced voltage determiner 512 and thedriving voltages Vα and Vβ input from the inverse coordinate converter505 to the induced voltage determiner 512, by using the followingequations:

Eα=Vα−R*iα−L*diα/dt, and  (7)

Eβ=Vβ−R*iβ−L*diβ/dt.  (8)

R is a winding resistance, and L is a winding inductance. The values ofthe winding resistance R and the winding inductance L are inherent tothe motor 509 in use, and stored in advance in the ROM 151 b or a memory(not illustrated) included in the motor control device 157.

The induced voltages Eα and Eβ determined by the induced voltagedeterminer 512 are output to a phase determiner 513.

Based on a ratio between the induced voltages Eα and Eβ output from theinduced voltage determiner 512, the phase determiner 513 determines therotation phase θ of the rotor 402 of the motor 509 by the followingequation:

θ=tan̂−1 (−Eβ/Eα).  (9)

In the present embodiment, the phase determiner 513 determines therotation phase θ by calculation based on equation (9). However, this isnot restrictive. For example, the phase determiner 513 may determine therotation phase θ by referring to a table showing a relationship betweenthe induced voltages Eα and Eβ and the rotation phase θ corresponding tothe induced voltages Eα and Eβ, stored in the ROM 151 b.

The rotation phase θ of the rotor 402 determined as described above isinput to the subtractor 101, the inverse coordinate converter 505, andthe coordinate converter 511.

The motor control device 157 repeats the foregoing control.

As described above, the motor control device 157 according to thepresent embodiment performs vector control using phase feedback controlfor controlling the current values in the rotating coordinate systemsuch that the deviation between the command phase θ_ref and the rotationphase θ decreases. By the vector control, the motor control device 157can suppress step-out of the motor 509 and an increase in motor noiseand power consumption due to surplus torque. The phase feedback controlcontrols the rotation phase θ of the rotor 402 such that the rotationphase θ of the rotor 402 becomes a desired phase. The image formingapparatus 100 thus applies vector control using phase feedback controlto the motor 509 that drives a load for which the rotation phase θ ofthe rotor 402 needs to be accurately controlled (for example,registration roller 308), whereby an appropriate image formation isperformed on a recording medium.

[Field Weakening]

Next, field weakening will be described. As mentioned above, therotation of the rotor 402 generates an induced voltage in the winding ofeach phase of the motor 509. The generation of the induced voltages inthe windings of the motor 509 reduces the voltages applicable to thewindings of the motor 509 (hereinafter, referred to as usable voltages).Specifically, suppose, for example, that the voltage value output fromthe power supply 1 is Vcc. Due to the generation of the induced voltagesin the windings of the respective phases, usable voltages Vα′ and Vβ′are limited to values given by the following equations (10) and (11):

Vα′=Vcc−eα, and  (10)

Vβ′=Vcc−eβ.  (11)

Here, eα represents the amplitude of the induced voltage Eα whichchanges sinusoidally, and eβ represents the amplitude of the inducedvoltage Eβ which changes sinusoidally.

The amplitude e of the induced voltage generated in the winding of eachphase by the rotation of the rotor 402 increases when the rotation speedof the rotor 402 increases. In other words, the higher the rotationspeed of the rotor 402, the lower the usable voltage. When the usablevoltage decreases, torque T that can be given to the rotor 402 alsodecreases.

The induced voltages are generated by a change in the magnetic fluxpassing through the windings. An increase in the magnitude of theinduced voltages occurring in the windings can thus be suppressed bycontrolling the exciting current component in such a way that a magneticflux weaker than that of the rotor 402 passes through the windings.Specifically, if the exciting current component is controlled so as tobe a negative value, the intensity of the magnetic flux of the rotor 402is apparently weakened, whereby a magnetic flux weaker than that of therotor 402 can be made to pass through the windings. This can suppress anincrease in the magnitude of the induced voltages occurring in thewindings, and can suppress a decrease in the usable voltages Vα′ andVβ′. As a result, a reduction in possible output torque can besuppressed. Such a technique is referred to as field weakening. Thegreater the absolute value of the negative value of the exciting currentcomponent, the more a reduction in the possible output torque can besuppressed.

Next, field weakening in the present embodiment will be described. Inthe present embodiment, the motor 509 is efficiently controlled by theapplication of the following configuration to the motor control device157.

The motor control device 157 according to the present embodimentperforms field weakening if the following two conditions are satisfied.

<Condition 1 for Performing Field Weakening>

The first condition for performing field weakening will initially bedescribed.

FIG. 7 is a graph illustrating a relationship between the torque T and arotation speed ω of the rotor 402. FIG. 7 illustrates a torqueT-rotation speed ω characteristic (broken line) when the d-axis currentis controlled to be zero and the torque T-rotation speed ωcharacteristic (full line) when the d-axis current is controlled to havea negative value. The torque T-rotation speed ω characteristicsillustrated in FIG. 7 are mere examples in the present embodiment, andare not restrictive.

As illustrated in FIG. 7, if the rotation speed ω is less than ω0(ω<ω0), the torque T when the d-axis current is controlled to be zero ishigher than the torque T when the d-axis current is controlled to have anegative value. In other words, if the rotation speed ω is less than ω0(ω<ω0), higher torque can be given to the rotor 402 without fieldweakening than with field weakening.

As illustrated in FIG. 7, if the rotation speed ω is higher than ω0(ω>ω0), the torque T when the d-axis current is controlled to have anegative value is higher than the torque T when the d-axis current iscontrolled to be zero. In other words, if the rotation speed ω is higherthan ω0 (ω>ω0), higher torque can be given to the rotor 402 with fieldweakening than without field weakening.

As described above, if the rotation speed ω is less than ω0 (ω<ω0),higher torque can be given to the rotor 402 without field weakening thanwith field weakening. If the rotation speed ω is higher than ω0 (ω>ω0),higher torque can be given to the rotor 402 with field weakening thanwithout field weakening.

In the present embodiment, the first condition for performing fieldweakening is therefore that the rotation speed ω of the rotor 402 is ω0or more.

As illustrated in FIG. 5, in the present embodiment, the CPU 151 acalculates a rotation speed ω_ref′ to substitute for a command speedω_ref based on the amount of change in the command phase θ_ref in apredetermined period, and outputs the rotation speed ω_ref′ to the fieldcontroller 540. The rotation speed ω_ref′ is calculated by using thefollowing equation (12):

ω=dθ/dt.  (12)

The field controller 540 determines whether the rotation speed ω_ref′satisfies the following expression (13):

ω_ref′≥ωth,  (13)

where ωth (=ω0) is a speed threshold which is stored in the memory 540a.

However, if expression (13) is the only condition for performing fieldweakening, an unneeded current can be supplied to the windings asdescribed above in a period when the motor 509 and the conveyance roller306 are not connected. This results in increased power consumption.

<Condition 2 for Performing Field Weakening>

Next, the second condition for performing field weakening will bedescribed.

As described above, in a period when the rotor 402 rotates at apredetermined speed, the load torque during a period in which the motor509 and the conveyance roller 306 are connected is higher than the loadtorque during a period in which the motor 509 and the conveyance roller306 are not connected. In other words, higher torque T is needed duringthe period in which the motor 509 and the conveyance roller 306 areconnected than during the period in which the motor 509 and theconveyance roller 306 are not connected.

As described in FIG. 2, the sheet sensor 330 for detecting the presenceor absence of a recording medium is provided in a predetermined positionupstream of the conveyance roller 306 in the conveyance direction inwhich the recording medium is conveyed. The sheet sensor 331 fordetecting the presence or absence of a recording medium is provided in asecond predetermined position downstream of the conveyance roller 306 inthe conveyance direction. The detection results of the sheet sensors 330and 331 are input to the CPU 151 a.

In the present embodiment, if the sheet sensor 330 detects the leadingedge of a recording medium (the leading edge of the recording mediumreaches the sheet sensor 330), the motor control device 157 starts fieldweakening. When a predetermined time t_on has elapsed from the detectionof the leading edge of the recording medium by the sheet sensor 330, theCPU 151 a controls the clutch 700 to connect the motor 509 and theconveyance roller 306. As a result, the motor 509 and the conveyanceroller 306 are connected with each other. If the trailing edge of therecording medium passes the sheet sensor 331 (the recording medium stopsbeing detected by the sheet sensor 331), the CPU 151 a controls theclutch 700 to separate the motor 509 and the conveyance roller 306. As aresult, the motor 509 and the conveyance roller 306 are separated fromeach other. When a predetermined time t_off has elapsed from the passingof the trailing edge of the recording medium by the sheet sensor 331,the motor control device 157 ends the field weakening. In other words,field weakening is performed in a period when the motor 509 and theconveyance roller 306 are connected. Field weakening is not performed ina period when the motor 509 and the conveyance roller 306 are notconnected. This can suppress an increase in power consumption due to thesetting of the value of the exciting current component to a value otherthan zero.

<Specific Method of Performing Field Weakening>

Next, a specific method of performing field weakening will be described.

FIG. 8 is a diagram illustrating a time chart of field weakening controlaccording to the present embodiment. During a period before the sheetsensor 330 detects the leading edge of a recording medium (period inwhich the sheet sensor 330 is ‘H (high level)’) in a period in which therotation speed ω_ref′ of the rotor 402 is ωth or more, the fieldcontroller 540 outputs 0 A as the d-axis current command value id_ref.In other words, field weakening is not performed.

If the sheet sensor 330 then detects the leading edge of a recordingmedium (the sheet sensor 330 changes from ‘H’ to ‘L (low level)’) in astate in which the rotation speed ω_ref′ is a predetermined speed, theCPU 151 a outputs a signal for switching the value of the d-axis currentcommand value id_ref to the field controller 540. According to theswitch signal, the field controller 540 switches the d-axis currentcommand value id_ref to a negative value (for example, −0.3 A) bygradually changing the d-axis current command value to be output from 0A. As a result, field weakening is performed. The d-axis current commandvalue id_ref is set to a predetermined value such that even if arecording medium (such as thick paper) that causes a maximum increase inload torque, among conveyable types of recording media is conveyed, theload torque will not exceed the possible output torque.

If the set value of the d-axis current command value id_ref is negativeand has too large an absolute value, the magnetic field occurring fromthe permanent magnet serving as the rotor 402 is weakened so excessivelythat the resulting torque occurring on the rotor 402 becomes low. If theset value of the d-axis current command value id_ref is negative and hasan absolute value close to zero, the magnetic field occurring from thepermanent magnet serving as the rotor 402 fails to be weakened and, as aresult, the induced voltages occurring in the windings fail to bereduced. In view of the foregoing, the negative value is determined inadvance based on experiments. The d-axis current command value id_ref isstored in the memory 540 a. The field controller 540 outputs the valuestored in the memory 540 a as the d-axis current command value id_ref.

When the predetermined time t_on has elapsed from the detection of theleading edge of the recording medium by the sheet sensor 330, the CPU151 a controls the clutch 700 to connect the motor 509 and theconveyance roller 306. As a result, the motor 509 and the conveyanceroller 306 are connected with each other.

If the recording medium is conveyed by the conveyance roller 306, thesheet sensor 331 detects the leading edge of the recording medium (thesheet sensor 331 changes from ‘H’ to ‘L’). If the recording medium isfurther conveyed and the trailing edge of the recording medium passesthe sheet sensor 331 (the sheet sensor 331 changes from ‘L’ to ‘H’), theCPU 151 a controls the clutch 700 to separate the motor 509 and theconveyance motor 306. As a result, the motor 509 and the conveyanceroller 306 are separated from each other.

When the predetermined time t_off has elapsed from the passing of thetrailing edge of the recording medium by the sheet sensor 331, the CPU151 a outputs the switch signal to the field controller 540. Accordingto the switch signal, the field controller 540 switches the d-axiscurrent command value id_ref to be output to 0 A by gradually changingthe d-axis current command value id_ref from the negative value (forexample, −0.3 A). As a result, field weakening ends.

The CPU 151 a then repeats the foregoing control.

In the present embodiment, it is defined by the sequence of the imageforming operation in advance to perform the connection and separation ofthe clutch 700 in a state where the rotation speed ω of the rotor 402 isa predetermined speed. In other words, it is defined by the sequence ofthe image forming operation in advance not to perform the connection orseparation of the clutch 700 while the rotor 402 is accelerating ordecelerating. In the present embodiment, the d-axis current commandvalue during the acceleration and deceleration of the rotor 402 istherefore zero.

The sheet sensor 330 is arranged in a position as close to theconveyance roller 306 as possible. This can reduce as much as possiblethe period in which field weakening is performed, and can suppress anincrease in power consumption. However, if the sheet sensor 330 is tooclose to the conveyance roller 306, the d-axis current command valueid_ref may fail to be switched in time for the connection of the motor509 and the conveyance roller 306. As a result, the load torque canexceed the possible output torque. The arranged position of the sheetsensor 330 and the predetermined time t_on are therefore set such thatthe d-axis current command value id_ref can be switched in time for theconnection of the motor 509 and the conveyance roller 306. Thepredetermined time t_on is stored in the ROM 151 b in advance.

The predetermined time t_off is set to as short a time as possible. Thiscan reduce as much as possible the period in which field weakening isperformed, and can suppress an increase in power consumption. However,if the predetermined time t_off is too short, field weakening may beended in a state where a change in the load torque occurring and actingon the rotor 402 due to the separation of the motor 509 and theconveyance roller 306 is relatively large. As a result, the load torquecan exceed the possible output torque. The arranged position of thesheet sensor 331 and the predetermined time t_off are therefore set suchthat field weakening will not be ended in the state where a change inthe load torque occurring and acting on the rotor 402 due to theseparation of the motor 509 and the conveyance roller 306 is relativelylarge. The predetermined time t_off is stored in the ROM 151 b inadvance.

FIG. 9 is a flowchart for describing a method of performing the fieldweakening control. The method of performing the field weakening controlwill be described below with reference to FIG. 9. The processing of theflowchart is executed by the CPU 151 a.

In step S1001, if the CPU 151 a outputs an enable signal ‘H’ to themotor control device 157 (YES in step S1001), the processing proceeds tostep S1002. The motor control device 157 thereby starts driving controlon the motor 509 based on a command output from the CPU 151 a. Theenable signal is a signal for enabling or disabling operation of themotor control device 157. If the enable signal is ‘L’ (NO in stepS1001), the processing proceeds to step S1001. That is, the CPU 151 adisables the operation of the motor control device 157. In other words,control of the motor 509 by the motor control device 157 is ended. Ifthe enable signal is ‘H’, the CPU 151 a enables the operation of themotor control device 157. The motor control device 157 performs drivingcontrol on the motor 509 based on a command output from the CPU 151 a.

In step S1002, the motor control device 157 performs vector controlbased on a command output from the CPU 151 a. In step S1003, if the CPU151 a outputs the enable signal ‘L’ to the motor control device 157 (YESin step S1003), the motor control device 157 ends driving the motor 509.In step S1003, if the CPU 151 a outputs the enable signal ‘H’ to themotor control device 157 (NO in step S1003), the processing proceeds tostep S1004.

In step S1004, the field controller 540 outputs 0 A as the d-axiscurrent command value id_ref. That is, field weakening is not performed.

In step S1005, if the rotation speed ω_ref′ is less than a speedthreshold ωth (YES in step S1005), the processing returns to step S1002to continue vector control. Field weakening is not performed here.

In step S1005, if the rotation speed ω_ref′ is the speed threshold ωthor more (NO in step S1005), the processing proceeds to step S1006.

In step S1006, if the sheet sensor 330 detects the leading edge of arecording medium (YES in step S1006), the processing proceeds to stepS1007. In step S1007, the CPU 151 a outputs the switch signal to thefield controller 540. According to the switch signal, the fieldcontroller 540 switches the d-axis current command value id_ref to beoutput from 0 A to a negative value (for example, −0.3 A). As a result,field weakening is performed.

In step S1008, if the predetermined time t_on has elapsed from thedetection of the leading edge of the recording medium by the sheetsensor 330 (YES in step S1008), the processing proceeds to step S1009.In step S1009, the CPU 151 a controls the clutch 700 to connect themotor 509 and the conveyance roller 306. As a result, the motor 509 andthe conveyance roller 306 are connected with each other.

In step S1010, the sheet sensor 331 detects the leading edge of therecording medium conveyed by the conveyance roller 306, and the sheetsensor 331 becomes ‘L’. In step S1011, if the trailing edge of therecording medium conveyed by the conveyance roller 306 passes the sheetsensor 331 (YES in step S1011), the processing proceeds to step S1012.In step S1012, the CPU 151 a controls the clutch 700 to separate themotor 509 and the conveyance roller 306. As a result, the motor 509 andthe conveyance roller 306 are separated from each other.

In step S1013, if the predetermined time t_off has elapsed from thepassing of the trailing edge of the recording medium by the sheet sensor331 (YES in step S1013), the processing proceeds to step S1014. In stepS1014, the CPU 151 a outputs the switch signal to the field controller540. According to the switch signal, the field controller 540 switchesthe d-axis current command value id_ref to be output from the negativevalue (for example, −0.3 A) to 0 A. As a result, field weakening ends.The processing then returns to step S1002 to continue vector control.

The motor control device 157 subsequently repeats the foregoing controlto control the motor 509 until the CPU 151 a outputs the enable signal‘L’ to the motor control device 157.

As described above, in the present embodiment, field weakening isperformed only during a period in which field weakening is needed.Specifically, field weakening is performed only during a period in whichthe motor 509 and the conveyance roller 306 are connected, in a periodwhen the rotation speed ω_ref′ is a predetermined speed higher than orequal to the speed threshold ωth. In the present embodiment, it isdefined by the sequence of the image forming operation in advance toperform the connection and separation of the clutch 700 in a state wherethe rotation speed ω_ref′ of the rotor 402 is the predetermined speed.In other words, it is defined by the sequence of the image formingoperation in advance not to perform the connection or separation of theclutch 700 while the rotor 402 is accelerating or decelerating. In thepresent embodiment, the d-axis current command value id_ref is thereforezero in a period when the rotation speed ω_ref′ of the rotor 402 islower than the predetermined speed, i.e., while the rotor 402 isaccelerating or decelerating. This can reduce the period during whichfield weakening is performed, in the period when the motor 509 isdriven, and can suppress an increase in power consumption. In otherwords, an increase in power consumption due to the setting of thepredetermined value of the exciting current component corresponding tothe predetermined speed in the period in which the rotor 402 is rotatedat the predetermined speed can be suppressed, and the motor 509 can beefficiently controlled.

In the present embodiment, the field controller 540 switches the d-axiscurrent command value id_ref by gradually changing the d-axis currentcommand value id_ref. However, this is not restrictive. For example, thefield controller 540 may directly switch the d-axis current commandvalue id_ref from 0 A to −0.3 A. The field controller 540 may directlyswitch the d-axis current command value id_ref from −0.3 A to 0 A.

In the present embodiment, the d-axis current command value id_ref inperforming field weakening is set to −0.3 A regardless of the sheet typeof the recording medium to be conveyed. However, this is notrestrictive. For example, the d-axis current command value id_ref inperforming field weakening may be set according to the sheet type of therecording medium to be conveyed.

In the present embodiment, the timing to control the clutch 700 isdetermined based on the detection results of the sheet sensors 330 and331. However, for example, the timing may be determined based on apreviously-set operation sequence of the image forming apparatus 100.The timing may be determined based on the number of pulses output to themotor 509. For example, the user may transmit an instruction to stopconveyance of the recording medium to the CPU 151 a by using theoperation unit 152, and the CPU 151 a may perform separation control onthe clutch 700 according to the instruction. Such a configuration canprevent the driving force of the motor 509 from being transmitted to theconveyance roller 306, whereby the conveyance of the recording mediumcan be stopped.

In the present embodiment, the d-axis current command values id_refstored in the memory 540 a are 0 A and −0.3 A. However, this is notrestrictive, and three or more values may be stored. In such a case, forexample, the CPU 151 a outputs a signal indicating which value to use tothe field controller 540. The field controller 540 switches the d-axiscurrent command value id_ref to be output based on the signal.

In the present embodiment, field weakening is controlled based on thedetection results of the sheet sensors 330 and 331. However, this is notrestrictive. For example, field weakening is started when apredetermined time T1 from a start of driving of the conveyance roller306 to predetermined timing before connection of the motor 509 and theconveyance roller 306 by the clutch 700 has elapsed. Field weakening isended when a predetermined time T2 from the start of driving of theconveyance roller 306 to second predetermined timing after separation ofthe motor 509 and the conveyance roller 306 by the clutch 700 haselapsed. Such a configuration may be employed. The predetermined timingand the second predetermined timing are determined based on apreviously-set operation sequence of the image forming apparatus 100.The predetermined timing and the second predetermined timing may bedetermined based on the number of pulses output to the motor 509.

In the present embodiment, the d-axis current command value id_ref isset to 0 A in periods other than that during which the motor 509 and theconveyance roller 306 are connected, in the period in which the rotationspeed ω_ref′ of the rotor 402 is a predetermined speed higher than orequal to the speed threshold ωth. However, the d-axis current commandvalue id_ref may be set to a value other than 0 A. Specifically, thed-axis current command value id_ref may be set to a value greater thanthe negative value that is set as the d-axis current commend valueid_ref during the period in which the motor 509 and the conveyanceroller 306 are connected, in the period in which the rotation speedω_ref′ of the rotor 402 is the predetermined speed higher than or equalto the speed threshold ωth. In other words, the magnetic flux passingthrough the windings in the periods other than that during which themotor 509 and the conveyance roller 306 are connected, in the period inwhich the rotation speed ω_ref′ is the predetermined speed higher thanor equal to the speed threshold ωth can be higher than the magnetic fluxpassing through the windings during the period in which the motor 509and the conveyance roller 306 are connected. An increase in powerconsumption can be effectively suppressed by setting the d-axis currentcommand value id_ref to a value as close to 0 A as possible.

In the present embodiment, the d-axis current command value id_ref isset to the value obtained by experiment in advance. However, this is notrestrictive. For example, the field controller 540 may be configured tochange the value of the d-axis current command value id_ref based on therotation speed ω_ref′ of the rotor 402. Specifically, the fieldcontroller 540 may be configured such that the higher the rotation speedω_ref′, the smaller the value of the d-axis current command value id_refis set to be. This can suppress an increase in the induced voltagesoccurring in the windings as the rotation speed ω_ref′ increases.

The d-axis current command value id_ref during field weakening is set tosuch a value that the rotation phase θ of the rotor 402 is accuratelydetermined even if the induced voltages occurring in the windingsdecrease because of the field weakening.

In the present embodiment, the field weakening control is applied to theconfiguration for connecting and separating the motor 509 and theconveyance roller 306 by using the clutch 700. However, the presentembodiment is not limited to a conveyance roller, and may be applied toa configuration for connecting and separating other loads.

In the present embodiment, the speed threshold ωth is set to ω0.However, this is not restrictive. For example, the speed threshold ωthmay be set to a value smaller than ω0. The speed threshold ωth may beset to a value greater than ω0.

In the present embodiment, a stepping motor is used as the motor 509 fordriving loads. However, other motors such as a direct-current (DC) motormay be used. The motor 509 is not limited to a two-phase motor. Thepresent embodiment may be applied to other motors such as a three-phasemotor.

In the vector control according to the present embodiment, the motor 509is controlled by performing phase feedback control. However, this is notrestrictive. For example, the rotation speed ω of the rotor 402 may befed back to control the motor 509. Specifically, as illustrated in FIG.10, the motor control device 157 includes a speed determiner 515. Thespeed determiner 515 determines the rotation speed ω based on a temporalchange in the rotation phase θ output from the phase determiner 513. Therotation speed ω is determined by using equation (12). The CPU 151 aoutputs the command speed ω_ref indicating the target speed of the rotor402. The motor control device 157 includes a speed controller 500, andthe speed controller 500 is configured to generate and output a q-axiscurrent command value iq_ref such that a deviation between the rotationspeed ω and the command speed ω_ref decreases. By such a speed feedbackcontrol, the motor 509 may be controlled. Since the rotation speed ω isfed back in this configuration, the rotation speed ω of the rotor 402can be controlled to a predetermined speed. The image forming apparatus100 thus applies the vector control using the speed feedback control toa motor that drives a load the rotation speed of which needs to becontrolled to a constant speed for the sake of appropriate imageformation on a recording medium. Examples of such a load includes thephotosensitive drum 309 and the conveyance belts 208 and 317. As aresult, appropriate image formation can be performed on the recordingmedium.

In the present embodiment, a permanent magnet is used as the rotor 402.However, this is not restrictive.

According to an embodiment, motor control can be efficiently performed.

While the present invention has been described with reference toembodiments, it is to be understood that the invention is not limited tothe disclosed embodiments. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. A sheet conveyance apparatus for conveying asheet, the sheet conveyance apparatus comprising: a conveyance rollerconfigured to convey the sheet; a motor configured to drive theconveyance roller; a clutch unit configured to switch between a firststate and a second state, wherein the first state is a state in which adriving force of the motor is transmitted to the conveyance roller, andthe second state is a state in which the driving force of the motor isnot transmitted to the conveyance roller; a phase determiner configuredto determine a rotation phase of a rotor of the motor; a detectorconfigured to detect a driving current flowing through a winding of themotor; and a controller configured to control the driving currentflowing through the winding of the motor so that a value of a torquecurrent component of the driving current detected by the detectorbecomes a target value of the torque current component and to performfield weakening for reducing an intensity of a magnetic flux passingthrough the winding by controlling the driving current flowing through awinding of the motor so that a value of an exciting current component ofthe driving current detected by the detector becomes a target value ofthe exciting current component, wherein the torque current component isa current component that generates torque on the rotor and is expressedin a rotating coordinate system based on the rotation phase determinedby the phase determiner, and the exciting current component is a currentcomponent that has an influence on the intensity of the magnetic fluxpassing through the winding and is expressed in the rotating coordinatesystem, wherein the controller sets the target value of the torquecurrent component so as to reduce a deviation between an instructedphase representing a target phase of the rotor and the rotation phasedetermined by the phase determiner, and wherein, in a period in whichthe rotor rotates at a predetermined speed, the controller sets thetarget value of the exciting current component such that a degree of thefield weakening in a first period after a first timing is greater than adegree of the field weakening in a second period before the firsttiming, wherein the first timing is earlier by first predetermined timethan switching of the motor and the conveyance roller from the secondstate to the first state.
 2. The sheet conveyance apparatus according toclaim 1, wherein the controller sets the target value of the excitingcurrent component such that a degree of the field weakening in a thirdperiod that is, in the first period, from the first timing to a secondtiming that is after a lapse of second predetermined time from switchingof the motor and the conveyance roller from the first state to thesecond state is greater than a degree of the field weakening in a fourthperiod after the second timing.
 3. The sheet conveyance apparatusaccording to claim 1, wherein the controller sets the target value ofthe excitation current component such that a degree of the fieldweakening during a period of rotation of the rotor at a speed lower thanthe predetermined speed is less than the degree of the field weakeningduring the first period.
 4. The sheet conveyance apparatus according toclaim 1, wherein the degree of the field weakening in a case where thevalue of the excitation current component is a first value is greaterthan the degree of the field weakening in a case where the value of theexcitation current component is a second value, and wherein the secondvalue is greater than the first value.
 5. The sheet conveyance apparatusaccording to claim 1, wherein, in a case where a rotation speed of therotor is less than a threshold, a magnitude of torque capable of beinggenerated on the rotor in a state where the value of the excitingcurrent component is a negative value is less than that of the torquecapable of being generated on the rotor in a state where the value ofthe exciting current component is zero, wherein, in a case where therotation speed of the rotor is more than the threshold, the magnitude ofthe torque capable of being generated on the motor in the state wherethe value of the exciting current component is the negative value isgreater than that of the torque capable of being generated on the rotorin the state where the value of the exciting current component is zero,and wherein the predetermined speed is higher than the threshold.
 6. Thesheet conveyance apparatus according to claim 1, further comprising: asecond controller configured to control the clutch unit; and a sheetdetector provided upstream of a nip of the conveyance roller in aconveyance direction, in which the sheet is conveyed by the conveyanceroller, and configured to detect whether there is the sheet or not,wherein the first timing is a timing at which the sheet detector detectsa leading edge of the sheet, and wherein the second controller controlsthe clutch unit such that the driving force of the motor is transmittedto the conveyance roller at a timing of a lapse of the firstpredetermined time from the first timing.
 7. The sheet conveyanceapparatus according to claim 1, wherein the controller sets the targetvalue of the excitation current component to zero for a period beforethe first timing in a period of rotation of the rotor.
 8. The sheetconveyance apparatus according to claim 1, wherein the controller setsthe target value of the excitation current component for the firstperiod in accordance with a rotation speed of the rotor.
 9. The sheetconveyance apparatus according to claim 1, wherein the controllerchanges the target value of the excitation current component graduallybetween a negative value and zero.
 10. A sheet conveyance apparatus forconveying a sheet, the sheet conveyance apparatus comprising: aconveyance roller configured to convey the sheet; a motor configured todrive the conveyance roller; a clutch unit configured to switch betweena first state and a second state, wherein the first state is a state inwhich a driving force of the motor is transmitted to the conveyanceroller, and the second state is a state in which the driving force ofthe motor is not transmitted to the conveyance roller; a phasedeterminer configured to determine a rotation phase of a rotor of themotor; a speed determiner configured to determine a rotation speed ofthe rotor; a detector configured to detect a driving current flowingthrough a winding of the motor; and a controller configured to controlthe driving current flowing through the winding of the motor so that avalue of a torque current component of the driving current detected bythe detector becomes a target value of the torque current component andto perform field weakening for reducing an intensity of a magnetic fluxpassing through the winding by controlling the driving current flowingthrough a winding of the motor so that a value of an exciting currentcomponent of the driving current detected by the detector becomes atarget value of the exciting current component, wherein the torquecurrent component is a current component that generates torque on therotor and is expressed in a rotating coordinate system based on therotation phase determined by the phase determiner, and the excitingcurrent component is a current component that has an influence on theintensity of the magnetic flux passing through the winding and isexpressed in the rotating coordinate system, wherein the controller setsthe target value of the torque current component so as to reduce adeviation between an instructed speed representing a target speed of therotor and the rotation speed determined by the speed determiner, andwherein, in a period in which the rotor rotates at a predeterminedspeed, the controller sets the target value of the exciting currentcomponent such that a degree of the field weakening in a first periodafter a first timing is greater than a degree of the field weakening ina second period before the first timing, wherein the first timing isearlier by first predetermined time than switching of the motor and theconveyance roller from the second state to the first state.
 11. A sheetconveyance apparatus for conveying a sheet, the sheet conveyanceapparatus comprising: a conveyance roller configured to convey thesheet; a motor configured to drive the conveyance roller; a clutch unitconfigured to switch between a first state and a second state, whereinthe first state is a state in which a driving force of the motor istransmitted to the conveyance roller, and the second state is a state inwhich the driving force of the motor is not transmitted to theconveyance roller; a phase determiner configured to determine a rotationphase of a rotor of the motor; a detector configured to detect a drivingcurrent flowing through a winding of the motor; and a controllerconfigured to control the driving current flowing through the winding ofthe motor so that a value of a torque current component of the drivingcurrent detected by the detector becomes a target value of the torquecurrent component and to perform field weakening for reducing anintensity of a magnetic flux passing through the winding by controllingthe driving current flowing through a winding of the motor so that avalue of an exciting current component of the driving current detectedby the detector becomes a target value of the exciting currentcomponent, wherein the torque current component is a current componentthat generates torque on the rotor and is expressed in a rotatingcoordinate system based on the rotation phase determined by the phasedeterminer, and the exciting current component is a current componentthat has an influence on the intensity of the magnetic flux passingthrough the winding and is expressed in the rotating coordinate system,wherein the controller sets the target value of the torque currentcomponent so as to reduce a deviation between an instructed phaserepresenting a target phase of the rotor and the rotation phasedetermined by the phase determiner, wherein, in a period in which therotor rotates at a predetermined speed, the controller sets the targetvalue of the exciting current component at a first value, which isnegative, for a first period after a first timing that is earlier byfirst predetermined time than switching of the motor and the conveyanceroller from the second state to the first state, and sets the targetvalue of the excitation current component at a second value, wherein thesecond value is greater than the first value, for a second period thatis before the first timing, and wherein a direction of a magnetic fluxgenerated because the value of the exciting current component isnegative is opposite of a direction of a magnetic flux of the rotor. 12.The sheet conveyance apparatus according to claim 11, wherein thecontroller sets the target value of the exciting current component atthe first value for a third period that is, in the first period, fromthe first timing to a second timing that is after a lapse of secondpredetermined time from switching of the motor and the conveyance rollerfrom the first state to the second state, and sets the target value ofthe exciting current component at a third value, which is greater thanthe first value, for a fourth period that is after the second timing.13. The sheet conveyance apparatus according to claim 11, wherein, for aperiod of rotation of the rotor at a speed lower than the predeterminedspeed, the controller sets the target value of the excitation currentcomponent at a value greater than the value set for the first period.14. The sheet conveying apparatus according to claim 11, wherein, in acase that a rotation speed of the rotor is less than a speed thresholdvalue, torque which can be generated in the rotor in a state in which avalue of the excitation current component is a negative value is lessthan or equal to torque which can be generated in the rotor in a statein which a value of the excitation current component is zero, wherein,in a case that a rotation speed of the rotor is greater than or equal tothe speed threshold value, torque which can be generated in the rotor ina state in which a value of the excitation current component is thenegative value is greater than torque which can be generated in therotor in the state in which a value of the excitation current componentis zero, and wherein the predetermined speed is a higher speed than thespeed threshold value.
 15. The sheet conveyance apparatus according toclaim 11, further comprising: a second controller configured to controlthe clutch unit; and a sheet detector provided upstream of a nip of theconveyance roller in a conveyance direction, in which the sheet isconveyed by the conveyance roller, and configured to detect whetherthere is the sheet or not, wherein the first timing is a timing at whichthe sheet detector detects a leading edge of the sheet, and wherein thesecond controller controls the clutch unit such that the driving forceof the motor is transmitted to the conveyance roller at a timing of alapse of the first predetermined time from the first timing.
 16. Thesheet conveyance apparatus according to claim 11, wherein the controllersets the target value of the excitation current component to zero for aperiod before the first timing in a period of rotation of the rotor. 17.The sheet conveyance apparatus according to claim 11, wherein thecontroller sets the target value of the excitation current component forthe first period in accordance with a rotation speed of the rotor. 18.The sheet conveyance apparatus according to claim 11, wherein thecontroller changes the target value of the excitation current componentgradually between a negative value and zero.
 19. A sheet conveyanceapparatus for conveying a sheet, the sheet conveyance apparatuscomprising: a conveyance roller configured to convey the sheet; a motorconfigured to drive the conveyance roller; a clutch unit configured toswitch between a first state and a second state, wherein the first stateis a state in which a driving force of the motor is transmitted to theconveyance roller, and the second state is a state in which the drivingforce of the motor is not transmitted to the conveyance roller; a phasedeterminer configured to determine a rotation phase of a rotor of themotor; a speed determiner configured to determine a rotation speed ofthe rotor; and a controller configured to control a driving currentflowing through a winding of the motor so that a value of a torquecurrent component of the driving current detected by a detector becomesa target value of the torque current component and to perform fieldweakening for reducing an intensity of a magnetic flux passing throughthe winding by controlling the driving current flowing through a windingof the motor so that a value of an exciting current component of thedriving current detected by the detector becomes a target value of theexciting current component, wherein the torque current component is acurrent component that generates torque on the rotor and is expressed ina rotating coordinate system based on the rotation phase determined bythe phase determiner, and the exciting current component is a currentcomponent that has an influence on the intensity of the magnetic fluxpassing through the winding and is expressed in the rotating coordinatesystem, wherein the controller sets the target value of the torquecurrent component so as to reduce a deviation between an instructedspeed representing a target speed of the rotor and the rotation speeddetermined by the speed determiner, wherein, in a period in which therotor rotates at a predetermined speed, the controller sets the targetvalue of the exciting current component at a first value, which isnegative, for a first period after a first timing that is earlier byfirst predetermined time than switching of the motor and the conveyanceroller from the second state to the first state, and sets the targetvalue of the excitation current component at a second value, wherein thesecond value is greater than the first value, for a second period thatis before the first timing, and wherein a direction of a magnetic fluxgenerated because the value of the exciting current component isnegative is opposite of a direction of a magnetic flux of the rotor. 20.A document reading apparatus for reading a document, the documentreading apparatus comprising: a document tray on which a document isstacked; a conveyance roller configured to convey the document stackedon the document tray; a reading unit configured to read the documentconveyed by the conveyance roller; a motor configured to drive a load; aclutch unit configured to switch between a first state and a secondstate, wherein the first state is a state in which a driving force ofthe motor is transmitted to the load, the second state is a state inwhich the driving force of the motor is not transmitted to the load; aphase determiner configured to determine a rotation phase of a rotor ofthe motor; a speed determiner configured to determine a rotation speedof the rotor; and a detector configured to detect a driving currentflowing through a winding of the motor; and a controller configured tocontrol the driving current flowing through the winding of the motor sothat a value of a torque current component of the driving currentdetected by the detector becomes a target value of the torque currentcomponent and to perform field weakening for reducing an intensity of amagnetic flux passing through the winding by controlling the drivingcurrent flowing through a winding of the motor so that a value of anexciting current component of the driving current detected by thedetector becomes a target value of the exciting current component,wherein the torque current component is a current component thatgenerates torque on the rotor and is expressed in a rotating coordinatesystem based on the rotation phase determined by the phase determiner,and the exciting current component is a current component that has aninfluence on the intensity of the magnetic flux passing through thewinding and is expressed in the rotating coordinate system, wherein thecontroller sets a target value of the torque current component so as toreduce a deviation between an instructed speed representing a targetspeed of the rotor and the rotation speed determined by the speeddeterminer, and wherein, in a period in which the rotor rotates at apredetermined speed, the controller sets the target value of theexciting current component such that a degree of the field weakening ina first period after a first timing is greater than a degree of thefield weakening in a second period before the first timing, wherein thefirst timing is earlier by first predetermined time than switching ofthe motor and the conveyance roller from the second state to the firststate.
 21. An image forming apparatus for forming an image on a printtarget medium, the image forming apparatus comprising: a conveyanceroller configured to convey the print target medium; an image formingunit configured to form an image on the print target medium conveyed bythe conveyance roller; a motor configured to drive a load; a clutch unitconfigured to switch between a first state and a second state, whereinthe first state is a state in which a driving force of the motor istransmitted to the load, the second state is a state in which thedriving force of the motor is not transmitted to the load; a phasedeterminer configured to determine a rotation phase of a rotor of themotor; a speed determiner configured to determine a rotation speed ofthe rotor; and a detector configured to detect a driving current flowingthrough a winding of the motor; and a controller configured to controlthe driving current flowing through the winding of the motor so that avalue of a torque current component of the driving current detected bythe detector becomes a target value of the torque current component andto perform field weakening for reducing an intensity of a magnetic fluxpassing through the winding by controlling the driving current flowingthrough a winding of the motor so that a value of an exciting currentcomponent of the driving current detected by the detector becomes atarget value of the exciting current component, wherein the torquecurrent component is a current component that generates torque on therotor and is expressed in a rotating coordinate system based on therotation phase determined by the phase determiner, and the excitingcurrent component is a current component that has an influence on theintensity of the magnetic flux passing through the winding and isexpressed in the rotating coordinate system, wherein the controller setsa target value of the torque current component so as to reduce adeviation between an instructed speed representing a target speed of therotor and the rotation speed determined by the speed determiner, andwherein, in a period in which the rotor rotates at a predeterminedspeed, the controller sets the target value of the exciting currentcomponent such that a degree of the field weakening in a first periodafter a first timing is greater than a degree of the field weakening ina second period before the first timing, wherein the first timing isearlier by first predetermined time than switching of the motor and theconveyance roller from the second state to the first state.